This application claims the priority of German patent document 199 55 608.3, filed Nov. 19, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an infrared (IR) camouflage device for land targets, especially suitable for camouflaging military objects, such as land vehicles, against thermal imaging devices and infrared guidance heads.
In thermal camouflage an attempt is made to adjust the heat radiation emitted by an object to be camouflaged to the level of the thermal background; that is, to influence the temperature of the observable surfaces by design measures, such as heat damping, isolation, and back ventilation. While improvements are known in the area of active signature (for example for internal heat sources, such as motor, drives, energy assemblies), no satisfactory solutions have been provided by these measures relative to solar (passive) heat signature, since the temperature response of military objects as a rule differs sharply from that of a natural background. Proposed solutions for compensating for these deviations by active heating or cooling are described for example in German patent document DE 32 17 977 A1 and are less practical, especially in view of their high energy consumption.
Other known techniques are aimed at achieving signature reduction, not by influencing the actual surface temperature, but by changing the emission behavior of the surface. It is known that heat radiation of a body is determined not only by its temperature but also by the thermal emissivity xcex5 of its surface. The use of low emission surface layers for infrared camouflage is known, and is described for example in German patent document DE 30 43 381 A1 and European patent document EP 0 123 660 A. One problem with this type of camouflage with low emission camouflage agents is that when the thermal emissivity xcex5 is reduced, the infrared reflectivity xcfx81 theoretically rises according to the formula xcfx81=1xe2x88x92xcex5. Hence, an increased reflection of ambient radiation takes place. This overlaps the natural emission so that the heat radiation (and therefore the observable radiation temperature when the thermal emissivity is reduced) depends to an increasing degree on the temperatures of the reflected ambient areas (ground temperature, sky temperature). Reflections from areas of the sky close to the zenith have proven to be especially critical since their radiation temperatures differ considerably, depending on the cloud cover, and can also heavily influence the signature. A known effect of low emission camouflage means is the observation of xe2x80x9ccold spotsxe2x80x9d, i.e., areas with a low radiation temperature compared with the background temperatures due to the reflection of cold areas of the sky.
In order to take this fact into account, European patent document EP 0 250 742 A1 discloses a device with which thermal emissivity can be controlled. The heat radiated from an object is adjusted within wide limits as desired with very slight energy demand by controlling the shares of heat reflection and emission. A substantial reduction of contrast of the thermal radiation relative to the background is possible. However, the high cost of making such systems and the necessity for additional measurement and regulating devices are disadvantages.
When using low emission infrared camouflage means, the geometric features of the object to be concealed have to be taken into account. Distinctions must be made in particular between:
Surfaces Areas inclined toward the ground
Surfaces that are horizontal oriented or inclined toward the sky
Surfaces that are vertical oriented or slightly inclined toward the sky (up to 25xc2x0 relative to vertical).
Basically, these areas require different embodiments of camouflaging means. For areas that are predominantly inclined to the ground, the known low emission camouflage agents with a permanent thermal emissivity that is as low as possible are used, since the ground temperatures located in front of the object will be reflected, independently of the observation point. The radiation temperature of the ground is generally identical to the rest of the thermal background. By transferring this temperature to the object to be camouflaged, a high reduction of contrast can be achieved with a corresponding camouflage effect. In this case, known LE (low emission) camouflaging agents can be used, such as for example, low emission paint (xe2x80x9cLEPxe2x80x9d) or low emission polymer foil (xe2x80x9cLEFxe2x80x9d).
For areas with predominantly horizontal alignment the known low emission camouflage agents cannot be used directly. The problem is that these areas, if they can be seen, always reflect predominantly sky temperatures near the zenith. Because these sky temperatures are very low, and can vary considerably depending on the cloud cover, reflected heat radiation is extremely dependent on the cloud cover. In many cases, horizontal areas that are provided with low emission camouflage means will be xe2x80x9ccold spotsxe2x80x9d when, by reflection of the cold sky, the natural emission is overcompensated. Low emission behavior is desired only to the extent that increasing solar heating of the surface necessitates a reduction of the thermal radiation.
Similar problems exist in areas that are directed upward (angle to the horizontal less than 65xc2x0). They can reflect the sky radiation as well.
The situation in camouflage of essentially vertical surfaces (this includes surfaces inclined slightly to the skyxe2x80x94up to 25xc2x0 relative to the vertical) is a mixture of the already-described conditions in the horizontal or the upwardly directed areas on the one hand and areas inclined to the ground on the other. Depending on the observation angle, the heat radiation reflected at the camouflage agent comes primarily from areas near the ground or from the sky radiation. The problem is that even small changes in the observation angle (or equivalently, small changes in inclination of the area, for example with moved camouflaged objects) can cause a sharp change in the radiation temperature of the object.
Hence, the object of the invention is to provide an effective camouflage device for object surfaces that are aligned essentially vertical without costly measurement and regulating devices.
Another object of the invention is to provide a camouflage arrangement in which the percentage of the reflected radiation that comes from sky radiation or ground radiation remains constant over a range of angles of inclination which is as large as possible.
These and other objects and advantages are achieved by the camouflage arrangement according to the invention, which breaks up the surface into areas inclined to the ground and areas inclined to the sky, with an area as large as possible of the radiation reflected at the camouflage device coming from the ground and the smallest percentage being reflected from the sky radiation or from only warmer sky areas near the horizon. This can be accomplished according to the invention by a surface structure that consists exclusively of two groups of partial areas, with the partial areas of the first group being directed downward and forming angles xcex1 between 5xc2x0 and 45xc2x0 with the vertical and the partial areas in the second group being aligned upward and forming angles xcex2 between 50xc2x0 and 85xc2x0 with the vertical, with xcex1+xcex2 being  less than 90xc2x0. The partial areas within each group can have different angles xcex1 and xcex2.
The total surface of all the upwardly aligned partial areas is advantageously less than the total area of all of the downwardly directed partial areas, and the structural sizes of the surface structure lie especially between 12 xcexcm and 1 cm, preferably between 100 xcexcm and 1 mm.
The structural sizes are chosen in an especially advantageous embodiment so that they are larger than the wavelength of infrared radiation and smaller than the wavelength of radar radiation. A size range suitable for the purpose is that between 20 xcexcm and 1 mm. This ensures that the radar radiation cross section will not be influenced negatively by the structure according to the invention.
To maintain the visual camouflage effect an IR-transparent cover layer (for example a pigmented and matted polyethylene foil) can be provided as an outer covering for the camouflage device in the direction of the observer.
Additional camouflage effects using the principle of spot camouflage paint can be achieved in which, even in the infrared, contour breakup is introduced. This can be produced very effectively by different thicknesses of the colored cover layer on top. Therefore, at all temperature states of the system, the infrared signature is superimposed by a spot-type pattern.
Advantageously, the downwardly directed partial areas that reflect the ground components are made of a material with a thermal emissivity that is as low as possible, i.e., maximum infrared reflectivity. (Typical values for this are xcex5xe2x89xa60.5.) The upwardly directed partial areas that reflect the sky radiation, on the other hand, can be made of a material with a high degree of infrared emission (especially xcex5xe2x89xa60.8, for example xcex5xe2x89xa60.9) so that the reflection of sky radiation can be suppressed.
The camouflage device according to the invention requires no additional control elements such as sensors, actuators, electronics, or cables. In addition, exact locally resolved determination of the surface temperature, which is present once the camouflage effect referred to at the outset according to the prior art for adjusting the thermal emissivity for each actively controllable IR camouflage element, is abent.
Further advantages of the device according to the invention are:
High IR camouflage efficiency is achieved for different objects;
The camouflage device according to the invention can be made in the form of sturdy, inexpensive elements;
Additional visual camouflage is possible in any desired color.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.